Lipid vesicles have been previously obtained, and observed through the use of radio-labeled liposomes, McDougall, I. R., Dunnick, J. K., McNamee, M. G., and Kriss, J. P. (1974) Proc. Natl. Acad. Sci. USA, 71 3487-3491; Hwang, K. J. and Mauk, M. R. (1977) Proc. Natl. Acad. Sci. USA, 74, 4991-4995; Hinkle, G. H., Born, G. S., Kessler, W. V., and Shaw, S. M. (1978) J. Phar. Sci. 67, 795-798. These vesicles contain relatively low levels of radioactive ions because of the limited amount of radioactive ions entrapped within the liposomes using simple sonication procedures. The internal aqueous volume of the vesicles is small with the result that only a few percent of the total suspension volume carrying the radioactive ions is encapsulated in the vesicle and the balance is lost for practical purposes.
The preferred ionophore (a generic term intended to imply compounds which are ion-loving or ion attracting) [6S-6.alpha.(2S*,3S*), 8.beta.(R*), 9.beta.,11.alpha.]-5-(methylamino)-2-[[3,9,11-trimethyl-8-[1-methyl-2-oxo- 2-(1H-pyrrol-2-yl)ethyl]-1,7-dioxasporo[5.5]undec-2-yl]methyl]-4-benzoxazol ecarboxylic acid, hereinafter referred to as ionophore A23187, has been used to complex and carry divalent cations across natural and artificial lipid membranes, Hyono, A., Hendriks, Th., Daemen, F. J. M., and Bonting, S. L. (1975) Biochim. Biophys. Acta., 389, 34-46; Sarkadi, B., Szasz, I., and Gardos, G. (1976) J. Membrane Biol., 26, 357-370; LaBelle, E. F. and Racker, E. (1977) J. Membrane Biol., 31, 301-315; Pfeiffer, D. R. Taylor, R. W. and Lardy, H. A. (1978) Ann. N.Y. Acad. Sci. (in press). Evidence also exists that A23187 can form complexes with trivalent cations, e.g., La.sup.+3 Pfeiffer, D. R., Reed, P. W., and Lardy, H. A. (1974) Biochemistry, 13, 4007-4014.
Gregoriadis and coworkers have labeled liposomes .sup.111 ln through use of .sup.111 ln-labeled bleomycin, Gregoriadis, G. and Neerunjun, E. D. (1975) Biochem. Biophys. Res. Comm, 65, 537-544; Gregoriadis, G. Neerunjun, D. E., and Hunt, R. (1977) Life Sci., 21 357-369. They reported 27-80% of the added radioactivity associated with the phospholipid in negatively charged liposomes and observed 2-4.5% incorporated into positively charged liposomes.
More recently, according to co-pending application Ser. No. 148,102 filed Feb. 22, 1979, there has been discovered a method for routinely loading radioactive ions into lipid vesicles with greater than 90% efficiency. In this method the ionophore is incorporated in the lipid bilayer and is used to carry externally added radioactive ions to a chelator or chelating agent, which was previously entrapped in the vesicles. The binding of the radioactive ion to the chelating agent is sufficiently strong that it provides the driving force for the net transfer of the radioactive ion into the vesicles. These radio-labeled vesicles show more than a 100-fold increase in specific activity over those loaded by simple sonication.
According to this invention, we have found that lipid vesicles wherein the bilayer wall includes cholesterol, distearoyl phospatidylcholine and the various cholesterol derivatives or hydrocarbon compounds with charged hydrophilic headgroups controllably release any drug, or other beneficial agent carried by or within the vesicle. Moreover, certain aminomannose and/or aminogalactose derivatives of cholesterol, have the characteristic upon injection into the host mammal of rapidly and preferentially accumulating in the lymphatic system and/or the lung, spleen or liver. Since these vesicles can be loaded with a wide variety of radioactive tracers and drugs, the vesicles of this invention provide a unique method of delivering a drug to the specific site of a disease. The vesicle wall remains substantially intact for several hours effectively to act as a carrier. Ultimately, the vesicle wall breaks down and releases the drug.